The present invention relates to methods for measuring the thickness of a layer in a multi-layered object.
There are several prior art devices for measuring the thickness of a layer in multi-layer materials.
U.S. Pat. Nos. 5,974,886 and 5,197,019 disclose a method in which a short pulse vibration generated by a transducer is directed to the material. Each interface between adjacent layers generates an echo that arrives at a detector transducer at different times. The thickness of a layer is calculated as the time difference between the two echoes formed at the two surfaces of the layer multiplied by the sound velocity in the layer. However, for thin layers the time between the echoes is very small, and any error in the time measurement leads to a corresponding error in the thickness. In addition, a crystal transducer usually has several lobes, (a main lobe and side lobes) and the pulse echo of the side lobes will be superimposed on the main pulse echoes and will thus add noise to the measurement.
Another method known in the art involves submerging the material in a coupler liquid and obtaining the frequency spectrum of the material, for example, as described in U.S. Pat. No. 5.351,544. This however cannot be used for in vivo measurement since it is impractical to introduce a coupler liquid into the body. U.S. Pat. No. 5,806,520 discloses a method for determining the thickness of bone, but this method is not accurate for hidden or hard to access tissues, or layers of small thickness.
The present invention provides an apparatus and method for measuring the thickness of hidden or hard to access layers in a multi-layer structure.
In accordance with the inventions an input vibrational wave is transmitted to the surface of the structure by means of a probe. The steady state echo of the input wave is the superimposition of a series of echoes formed at the front and back surfaces of the layer. The steady state echo is transmitted from the structure to a detector through the probe that determines the energy of the steady state echo and stores it in a memory. The frequency of the generated wave is varied, and the energy of the steady state echo at each frequency is determined and stored in the memory. As described in detail below, the thickness of the layer is then calculated from frequencies at which the intensity of the steady state echo is minimal. The invention may be used to determine the thickness of a layer in an organism. For example, the invention may be used to determine the thickness of a bone.
The invention allows non-invasive measurement of a layer thickness and may therefore be used in medical imaging procedures. The invention may be used to measure the thickness of hidden or difficult to access structures, such as bone or arteriosclerosis in an artery.
Thus, in its first aspect, the invention provides a method for determining a thickness x1 of a layer in an object, the method comprising the steps of:
(a) for each of a plurality of frequencies f1, . . . fk 
(aa) generating a continuous vibrational wave at a surface of the layer;
(ab) measuring an energy of a steady state echo wave produced in the object in response to the generated vibrational wave;
(b) calculating the thickness of the layer based upon the measured energies of the steady state echo waves.
In its second aspect, the invention provides a method a method for detecting the thickness of a bone, in an organism the method comprising the steps of:
(a) for each of a plurality of frequencies f1, . . . fk 
(aa) generating a continuous vibrational wave at a surface of the bone;
(ab) measuring an energy of a steady state echo wave produced in the organism in response to the generated vibrational wave;
(b) calculating the thickness of the bone based upon the measured energies of the steady state echo waves.
In its third aspect, the invention provides a device for determining a thickness of a layer in an object, the device comprising:
(a) a transducer configured to generate a plurality of input vibrational wave pulses;
(b) a receiver configured to receive a steady-state echo wave produced by an input vibrational wave pulse;
(c) a probe configured to transmit a vibrational wave from the transducer to a surface of the bone and to transmit steady-state echo wave from the surface to the receiver;
(d) a processor configured to
(da) determine a frequency of each of the plurality of input vibrational waves;
(db) store in a memory an. energy of each of a plurality of steady-state echo waves; and
(dc) calculate the thickness based upon the stored energies of the steady-state echo waves; and
(e) a display configured to display the thickness.
In its fourth aspect, the invention provides a device for determining a thickness of a bone in an organism, the device comprising:
(a) a transducer configured to generate a plurality of input vibrational wave pulses;
(b) a receiver configured to receive a steady-state echo wave produced by an input vibrational wave pulse;
(c) a probe configured to transmit a vibrational wave from the transducer to a surface of the bone and to transmit steady-state echo wave from the surface to the receiver;
(d) a processor configured to
(da) determine a frequency of each of the plurality of input vibrational waves;
(db) store in. a memory an energy of each of a plurality of steady-state echo waves; and
(dc) calculate the thickness based upon the stored energies of the steady-state echo waves; and
(e) a display configured to display the thickness.